Global climate is critically sensitive to physical and biogeochemical dynamics in the subpolar Southern Ocean, since this is where the deepest, most carbon-rich layers of the world ocean outcrop and exchange carbon with the atmosphere. Here, we show that the conventional framework for the subpolar Southern Ocean carbon cycle, which attributes a dominant role to the vertical overturning circulation and shelf-sea processes, fundamentally misrepresents regional carbon uptake. Observations in the Weddell Gyre – a key representative region of the subpolar Southern Ocean – show that the rate of carbon uptake is in fact set by an interplay between the Gyre’s horizontal circulation and the remineralisation at mid-depths of organic carbon sourced from open-ocean biological production. These results demonstrate that reframing the carbon cycle of the subpolar Southern Ocean is an essential step to better define its role in past and future climate change.